Study On The Synthesis And Electrochemical Performance Of LiFePO₄ Cathode For Lithium Ion Batteries

The olivine LiFePO4, which offers benefits such as a quite large theoretical capacity, flat discharge curve, good cycle stability, thermal and structural stability, inexpensive cost and environmental benignity, has been attracting much attention as a promising new cathode material for lithium-ion batteries. However, it was reported that this cathode has very low electronic conductivity and diffusion-controlled kinetics of the electrochemical process, which decrease utilization the charge-discharge capacity of active material. Therefore, efforts were devoted to the increase of electron conductivity and the decrease of dynamics limitation for lithium ions insertion/extraction in LiFePO₄ material. In this dissertation, solid-state reaction, carbothermal reduction method, sol-gel method and microemulsion method were adopted to synthesize LiFePO₄/C composite cathodic material. The effect of synthetic method and synthetic conditions on the particles size, morphology and electrochemical performance of LiFePO₄ products were reasonedly studied. AC impedance technique was used to investigate the dynamics process of the Li⁺ insertion/extraction in LiFePO₄ material and obtained some meaningful results. The thesis is divided into six chapters.In the first chapter, the current survey on the lithium ion batteries cathodic materials, especially LiFePO₄ material, was detailedly introduced and correlative scientific problems were brought forward.In the second chapter, the structural similarity between reactants and product, which can decrease the ions rearrangement energy and favor the formation of product cores, was guided to select Li₃PO₄ as reactants for the synthesis of target LiFePO₄. The effect of the acetylene black amount, the protective ambience category and the sintering temperature on the contents of Fe²⁺ and the electrochemical performance of final products was investigated carefully.In the third chapter, a half-solid-state carbothermal reduction reaction was used to prepare LiFePO₄/C. This synthetic route seem ideally suited to coating carbon on the surface of LiFePO₄ particles more homogeneously, increasing the electron conductivity of LiFePO₄ material, favoring stabilization of iron as Fe²⁺ and obtaining LiFePO₄ product with good electrochemical performance. The effects of different carbon sources, solid state method and half-solid-state method on the mechanism of reaction and particles size controlling were discussed in detail.In the fourth chapter, optimal conditions for the sol-gel synthesis of LiFePO₄ by using citric acid as chelant were investigated. Polyethylene glycol (PEG) was added in the sol-gel process to improve conglomeration of the product particles and wide distribution of particles size, and obtain LiFePO₄ material with better electrochemical performance. The influence of PEG on the mechanism of particles size and morphology controlling was detailedly discussed.In the fifth chapter, an n-octane/n-butyl/cetyltrimethyl ammonium bromide microemulsion system was firstly selected to synthesize successfully LiFePO₄/C composite. The incorporation between water cores in emulsion system, which induced the inhomogeneous particles size and size distribution of the final product, was overcome by adding PEG4000 and the improved electrode performance was obtained compared with other emulsion methods. The relationship between surfactant PEG4000 added into the microemulsion and the product particles size and morphology was discussed in detail.In the last chapter, AC impedance technique was used to study dynamics process of lithium insertion/extraction in LiFePO₄ material. The apparent activation energy of the lithium ions difiEusion and the apparent activation energy of the charge transfer were calculated for the samples synthesized by solid state method and microemulsion method, respectively. The calculation indicated that the lithium ions diffusion process was not enslaved to the electron conductivity but to the lithium ion conductivity when electron conductivity was largely increased by coating carbon on the surface of LiFePO₄ material. Therefore, lithium ions conductivity was suggested a key factor for the farther enhancement of electrochemical performance of LiFePO₄ material. At the same time, some feasible approaches were suggested increasing the lithium ions conductivity.